Description

glReadPixels returns pixel data from the frame buffer,
starting with the pixel whose lower left corner
is at location (x, y),
into client memory starting at location data.
Several parameters control the processing of the pixel data before
it is placed into client memory.
These parameters are set with glPixelStore.
This reference page describes the effects on glReadPixels of most,
but not all of the parameters specified by these three commands.

If a non-zero named buffer object is bound to the GL_PIXEL_PACK_BUFFER target
(see glBindBuffer) while a block of pixels is
requested, data is treated as a byte offset into the buffer object's data store
rather than a pointer to client memory.

glReadPixels returns values from each pixel with lower left corner at
x+iy+j
for
0<=i<width
and
0<=j<height.
This pixel is said to be the
ith
pixel in the
jth
row.
Pixels are returned in row order from the lowest to the highest row,
left to right in each row.

format specifies the format for the returned pixel values;
accepted values are:

GL_STENCIL_INDEX

Stencil values are read from the stencil buffer.
Each index is converted to fixed point.

GL_DEPTH_COMPONENT

Depth values are read from the depth buffer.
Each component is converted to floating point such that the minimum depth
value maps to 0 and the maximum value maps to 1.
Each component is clamped to the range
01.

GL_DEPTH_STENCIL

Values are taken from both the depth and stencil buffers. The type parameter
must be GL_UNSIGNED_INT_24_8 or GL_FLOAT_32_UNSIGNED_INT_24_8_REV.

GL_RED

GL_GREEN

GL_BLUE

GL_RGB

GL_BGR

GL_RGBA

GL_BGRA

Finally, the indices or components
are converted to the proper format,
as specified by type.
If format is GL_STENCIL_INDEX
and type is not GL_FLOAT,
each index is masked with the mask value given in the following table.
If type is GL_FLOAT, then each integer index is converted to
single-precision floating-point format.

If format is
GL_RED,
GL_GREEN,
GL_BLUE,
GL_RGB,
GL_BGR,
GL_RGBA, or
GL_BGRA and type is not GL_FLOAT,
each component is multiplied by the multiplier shown in the following table.
If type is GL_FLOAT, then each component is passed as is
(or converted to the client's single-precision floating-point format if
it is different from the one used by the GL).

type

Index Mask

Component Conversion

GL_UNSIGNED_BYTE

28-1

28-1⁢c

GL_BYTE

27-1

28-1⁢c-12

GL_UNSIGNED_SHORT

216-1

216-1⁢c

GL_SHORT

215-1

216-1⁢c-12

GL_UNSIGNED_INT

232-1

232-1⁢c

GL_INT

231-1

232-1⁢c-12

GL_HALF_FLOAT

none

c

GL_FLOAT

none

c

GL_UNSIGNED_BYTE_3_3_2

2N-1

2N-1⁢c

GL_UNSIGNED_BYTE_2_3_3_REV

2N-1

2N-1⁢c

GL_UNSIGNED_SHORT_5_6_5

2N-1

2N-1⁢c

GL_UNSIGNED_SHORT_5_6_5_REV

2N-1

2N-1⁢c

GL_UNSIGNED_SHORT_4_4_4_4

2N-1

2N-1⁢c

GL_UNSIGNED_SHORT_4_4_4_4_REV

2N-1

2N-1⁢c

GL_UNSIGNED_SHORT_5_5_5_1

2N-1

2N-1⁢c

GL_UNSIGNED_SHORT_1_5_5_5_REV

2N-1

2N-1⁢c

GL_UNSIGNED_INT_8_8_8_8

2N-1

2N-1⁢c

GL_UNSIGNED_INT_8_8_8_8_REV

2N-1

2N-1⁢c

GL_UNSIGNED_INT_10_10_10_2

2N-1

2N-1⁢c

GL_UNSIGNED_INT_2_10_10_10_REV

2N-1

2N-1⁢c

GL_UNSIGNED_INT_24_8

2N-1

2N-1⁢c

GL_UNSIGNED_INT_10F_11F_11F_REV

--

Special

GL_UNSIGNED_INT_5_9_9_9_REV

--

Special

GL_FLOAT_32_UNSIGNED_INT_24_8_REV

none

c (Depth Only)

Return values are placed in memory as follows.
If format is
GL_STENCIL_INDEX,
GL_DEPTH_COMPONENT,
GL_RED,
GL_GREEN, or
GL_BLUE,
a single value is returned and the data for the
ith
pixel in the
jth
row
is placed in location
j⁢width+i.
GL_RGB and GL_BGR return three values,
GL_RGBA and GL_BGRA return four values for each pixel,
with all values corresponding to a single pixel occupying contiguous space
in data.
Storage parameters set by glPixelStore,
such as GL_PACK_LSB_FIRST and GL_PACK_SWAP_BYTES,
affect the way that data is written into memory.
See glPixelStore for a description.

Notes

Values for pixels that lie outside the window
connected to the current GL context are undefined.

If an error is generated,
no change is made to the contents of data.

Errors

GL_INVALID_ENUM is generated if format or type is not an
accepted value.

GL_INVALID_VALUE is generated if either width or height is negative.

GL_INVALID_OPERATION is generated if format is GL_STENCIL_INDEX
and there is no stencil buffer.

GL_INVALID_OPERATION is generated if format is GL_DEPTH_COMPONENT
and there is no depth buffer.

GL_INVALID_OPERATION is generated if format is GL_DEPTH_STENCIL
and there is no depth buffer or if there is no stencil buffer.

GL_INVALID_ENUM is generated if format is GL_DEPTH_STENCIL
and type is not GL_UNSIGNED_INT_24_8 or GL_FLOAT_32_UNSIGNED_INT_24_8_REV.

GL_INVALID_OPERATION is generated if type is one of
GL_UNSIGNED_BYTE_3_3_2,
GL_UNSIGNED_BYTE_2_3_3_REV,
GL_UNSIGNED_SHORT_5_6_5, or
GL_UNSIGNED_SHORT_5_6_5_REV
and format is not GL_RGB.

GL_INVALID_OPERATION is generated if type is one of
GL_UNSIGNED_SHORT_4_4_4_4,
GL_UNSIGNED_SHORT_4_4_4_4_REV,
GL_UNSIGNED_SHORT_5_5_5_1,
GL_UNSIGNED_SHORT_1_5_5_5_REV,
GL_UNSIGNED_INT_8_8_8_8,
GL_UNSIGNED_INT_8_8_8_8_REV,
GL_UNSIGNED_INT_10_10_10_2, or
GL_UNSIGNED_INT_2_10_10_10_REV
and format is neither GL_RGBA nor GL_BGRA.

GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to the
GL_PIXEL_PACK_BUFFER target and the buffer object's data store is currently mapped.

GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to the
GL_PIXEL_PACK_BUFFER target and the data would be packed to the buffer
object such that the memory writes required would exceed the data store size.

GL_INVALID_OPERATION is generated if a non-zero buffer object name is bound to the
GL_PIXEL_PACK_BUFFER target and data is not evenly divisible
into the number of bytes needed to store in memory a datum indicated by type.

GL_INVALID_OPERATION is generated if GL_READ_FRAMEBUFFER_BINDING
is non-zero, the read framebuffer is complete, and the value of GL_SAMPLE_BUFFERS
for the read framebuffer is greater than zero.